1. Field of the Invention
The present invention relates to an adhesive composition which is used to attach a semiconductor chip die on a lead frame paddle in production of semiconductor integrated circuit package. More particularly, the present invention is concerned with a copper oxide-filled polymer die attach adhesive composition, capable of maintaining electric conductivity under severe temperature cycle and providing bonding strength enough to prevent package crack and delamination.
2. Description of the Prior Art
As a die attach adhesive paste for improvement in both electrical contact with die backside and electric conductivity which are necessary in a device such as power transistor, there has usually been used a silver-filled die attach adhesive composition. Such typical die attach adhesive composition comprises a base resin such as epoxy monomers, an amine-based curing agent, a diluent such as epoxide or butyl carbitol acetate, an adhesion promoter such as silane or dicyandiamide, a thixotropic agent such as colloidal magnesium silicate, and a small amount of additives such as Si, exhibiting improved electric and thermal conductivity through the inclusion of 60 to 80% by weight of inorganic filler of silver (Ag). In case of tape type, the die attach adhesive comprises a resin mixture of phenol and nitrile-butadien rubber and an inorganic filler, such as silver.
Silver, an electrical and thermal conductive inorganic filler for such adhesive composition, is usually added in a form of flake by virtue of its superior dispersion in organic resin systems. The silver flakes can give good thixotropic control when carrying out dispersion or die placement. However, in case of silver-filled adhesive paste, characteristic fluidity of silver cannot keep electric conductivity stable when subjected to temperature cycle (T/C), so that electric resistance is extremely varied. Referring to FIG. 3, there are plotted electric resistance values of silver flake 30 with regard to time under T/C -65.degree. C./150.degree. C. As shown in this figure, the electric resistance of silver flake is extremely varied owing to its characteristic fluidity, which incapacitates electric conductivity from being kept stable in the silver-filled adhesive paste.
In addition, the silver-filled adhesive paste cannot provide sufficient bonding strength for the die and lead frame paddle, causing package cracks and delamination in semiconductor package. Accordingly, conventional silver-filled adhesive paste has a serious problem that the semiconductor package is lowered in reliability.
In the case that the lead frame is made of copper, since the coefficient of thermal expansion (hereinafter referred to as "CTE") of the lead frame paddle makes little difference from that of epoxy molding compound (hereinafter referred to as "EMC"), most of post mold cure (hereinafter referred to as "PMC") stresses are concentrated in the interface between the silicon chip and the lead frame paddle which are significantly mismatched with each other in CTE. The following Table 1 lists several physical properties in various materials.
TABLE 1 ______________________________________ Young's Module Poisson's Coeff. of Thermal Material GPa Ratio Expansion .alpha.(ppm/.degree.C.) ______________________________________ Silicon 188 0.3 3 Alloy 42 148 0.3 5 Copper 113 0.33 17 EMC 14.7 0.25 18 ______________________________________
Accordingly, if the concentrated stress exceeds the bonding strength provided by adhesive to the interface between the silicon chip and the lead frame paddle, there occurs delamination. In addition, moisture in the air permeates the delaminated parts and is condensed therein until it is suddenly vaporized upon infrared reflow or vapor phase soldering. The sudden vaporization, called "pop corn phenomenon", causes package crack which deleteriously affects reliability of semiconductor package.
Referring to FIG. 1, there is shown a conventional lamination structure of semiconductor integrated circuit package 10. As shown in this figure, a semiconductor chip 11 is mounted on a lead frame paddle 12 through a die attach adhesive 14 with an EMC 15 encompassing all of them.
It is generally recognized that bonding strength is relatively weak at an interface between the EMC and a fillet portion 16 of silver-containing adhesive. This is because silver dispersed on the surface of the fillet cannot make a chemical bond with an epoxy ring ##STR1## of EMC. As a result, not only is good adhesion unaccomplished, but also PMC marginal delamination is apt to occur.
In addition, since the fillet portions are located at a corner and an edge of the die which both are stressed with more shear stress than any other part of the die, the delamination caused by CTE mismatch is highly prone to occur at the fillet. Such delaminated portions are permeated with the moisture of the atmosphere, to cause cracks along a wall of the chip on a surface of the package or package cracks along the lead frame paddle.